Deck 24: Expansion Devices

Question 1

The solid-state controlled step-motor valve has a temperature sensor, or thermistor, as one of its inputs .

Answer 1

True

Question 2

The following is a function of an expansion device:  ____.

A) produce a reduction in the pressure on the liquid refrigerant
B) regulate the flow of refrigerant into the evaporator
C) lower the boiling point of the refrigerant far enough below the temperature of the load to get the needed heat transfer rate
D) All of the above.

Answer 2

D

Question 3

The cross liquid charge bulb at the TXV contains fluid that ____.

A) has similar characteristics to the system fluid to prevent liquid refrigerant from entering the suction line
B) has different characteristics from the system fluid
C) causes the valve to stay open during the off cycle
D) follows the system's pressure-temperature relationship

Answer 3

B

Question 4

The purpose of a capillary tube is to ____.

A) regulate the refrigerant flow into the evaporator
B) lower the pressure of the liquid refrigerant
C) lower the boiling point of the refrigerant
D) All of the above.

Question 5

The step-motor valve uses a small motor to control the valve's port, which in turn controls evaporator superheat.

Question 6

Pressure entering the compressor that is too high will cause the density of suction gases entering the compressor to increase .

Question 7

When the load on the evaporator increases, the TXV ____.

A) increases the flow of refrigerant
B) decreases the flow of refrigerant
C) maintains constant head pressure
D) decreases the pressure drop across the evaporator

Question 8

The pressure-limiting TXV is desirable in low temperature applications because it keeps the suction pressure to the compressor down during hot pulldowns .

Question 9

The pin or valve port piston of the electronic step-motor expansion valve can move in a linear motion of 0.0000783 in./step .

Question 10

If the thermal bulb assembly develops a leak, what will occur?

A) All of the system's refrigerant will leak out.
B) The bulb pressure will reduce and the valve will close.
C) The valve will open to allow more refrigerant to flow.
D) It will have no effect on system operation.

Question 11

The TXV sensing bulb is mounted ____.

A) on the liquid line between the condenser and the TXV
B) at the inlet to the condenser
C) on the suction line between the evaporator and the compressor
D) on the compressor discharge line

Question 12

AXVs are usually found on smaller equipment with somewhat constant evaporator loads .

Question 13

A thermostatic expansion valve meters refrigerant flow to the evaporator based on the ____.

A) rate of liquid vaporization
B) evaporator suction line superheat
C) external equalizer pressure
D) All of the above.

Question 14

The purpose of an external equalizer in a TXV is to ____.

A) compensate for excessive evaporator pressure drop
B) allow system to equalize faster
C) prevent the expansion valve from frosting up
D) allow refrigerant to expand in the compressor

Question 15

If a thermostatic expansion valve's thermal bulb came loose from the suction line at the outlet of the evaporator, which of the following could result?

A) The bulb pressure would increase and the thermostatic expansion valve would open.
B) The superheat in the evaporator would decrease.
C) The evaporating pressure and the compressor's suction pressure would increase .
D) All of the above are correct.

Question 16

A capillary tube metering device controls the refrigerant flow by pressure drop .

Question 17

An automatic expansion valve uses a thermal bulb to help control the amount of refrigerant being fed into the evaporator.

Question 18

When the evaporator pressure drops, the automatic expansion valve will open to allow more refrigerant to enter the evaporator .

Question 19

The pressure-limiting TXV ____.

A) allows the compressor to operate at low condenser entering air temperatures
B) shuts off the condenser fan during hot pull down
C) maintains the full refrigerant flow rate during a hot pulldown
D) keeps the suction pressure to the compressor down during a hot pulldown

Question 20

Balanced port TXVs are used to ____ ambient temperatures .

A) feed refrigerant at a normal rate with low
B) feed refrigerant at a normal rate with high
C) stop refrigerant flow with high
D) stop refrigerant flow with low

Question 21

What two pressures control the operation of an automatic expansion valve?

Question 22

When is an external equalizer-type TXV required?

Question 23

The automatic expansion valve maintains a constant ____________________ in the evaporator .

Question 24

Why should the TXV be adjusted to maintain a low superheat ?

Question 25

The TXV maintains a constant ____________________ in the evaporator .

Question 26

As the evaporator load increases, an automatic expansion valve will ____________________ to maintain the suction pressure at the set point .

Question 27

Explain why the external equalizer line on a thermostatic expansion valve should always be installed after (downstream of) the thermal bulb.

Question 28

The two pressures that push to close a thermostatic expansion valve are the ____________________ pressure and the evaporator pressure.

Question 29

Explain the recommended method of charging a system with a capillary tube system.